Blood oximeters, especially pulse oximeters are widely used for measuring oxygenation of blood of a patient since they provide a simple non-invasive method for monitoring the percentage of hemoglobin which is saturated with oxygen. Continuous monitoring of oxygen saturation via pulse oximetry is a standard of care for use in operating rooms, post anesthesia care units, critical care units, emergency departments etc.
A pulse oximeter typically comprises two small light-emitting diodes that emit light of different wavelengths, typically in the red and the infrared part of the spectrum, respectively. The part of the emitted light transmitted through or reflected by tissue of a part of the patient's body, typically a fingertip or an ear lobe, is collected with a photodiode. Since absorption of these different wavelengths differs between oxyhemoglobin and its deoxygenated form, from the ratio of the collected red and infrared light, respectively, the percentage of hemoglobin which is saturated with oxygen can be determined. Such a pulse oximeter is known from U.S. Pat. No. 5,595,176.
It is an essential feature of conventional pulse oximeters that they rely on the pulsed part of the collected signal in order to discriminate the pulsating blood flow from the static tissue. Accordingly, pulse oximetry performs pure at low blood perfusion. Further, involuntary patient movements can be cumbersome for such measurements leading to poor measuring accuracy or defective measurement results.